Add a callback to inform embedders when an isolate is being shut down.

vm/dart.h

Index: vm/dart.h
===================================================================
--- vm/dart.h	(revision 7933)
+++ vm/dart.h	(working copy)
@@ -19,7 +19,8 @@
 class Dart : public AllStatic {
  public:
   static bool InitOnce(Dart_IsolateCreateCallback create,
-                       Dart_IsolateInterruptCallback interrupt);
+                       Dart_IsolateInterruptCallback interrupt,
+                       Dart_IsolateShutdownCallback shutdown);
 
   static Isolate* CreateIsolate(const char* name_prefix);
   static RawError* InitializeIsolate(const uint8_t* snapshot, void* data);